Known in the art is an internal combustion engine arranging in an engine exhaust passage an NOx storing catalyst which stores NOx contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releases stored NOx when the air-fuel ratio of the inflowing exhaust gas is a stoichiometric air-fuel ratio or rich, arranging downstream of the NOx storing catalyst an NOx selective reducing catalyst capable of selectively reducing the NOx in the exhaust gas with adsorbed ammonia, arranging upstream of the NOx storing catalyst a three-way catalyst, and temporarily switching the air-fuel ratio of the exhaust gas flowing into the three-way catalyst from lean to rich to release NOx from the NOx storing catalyst (see Japanese Patent Publication (A) No. 11-30117).
In this internal combustion engine, when the air-fuel ratio of the exhaust gas is temporarily switched from lean to rich to release NOx from the NOx storing catalyst, there are cases of a comparatively large amount of ammonia being generated at the three-way catalyst, and the ammonia not used for the reduction of the NOx at the NOx storing catalyst at this time flowing out from the NOx storing catalyst. In this case, the ammonia flowing out from the NOx storing catalyst is adsorbed on the NOx selective reducing catalyst.
On the other hand, in this internal combustion engine, there are cases of a slight amount of NOx not stored at the NOx storing catalyst flowing out from the NOx storing catalyst during normal operation wherein the exhaust gas is maintained at a lean air-fuel ratio. However, in this internal combustion engine, when NOx flows out from the NOx storing catalyst in this way, this NOx is reduced by the ammonia adsorbed on the NOx selective reducing catalyst. From another angle, the ammonia adsorbed on the NOx selective reducing catalyst is consumed to reduce the NOx and thereby removed from the NOx selective reducing catalyst.
However, in this internal combustion engine, the outflow of NOx from the NOx storing catalyst occurs in the general course of things. Accordingly, if a state of NOx not flowing out from the NOx storing catalyst continues, the ammonia adsorbed on the NOx selective reducing catalyst continues to be adsorbed on the NOx selective reducing catalyst without reacting with the NOx and being removed. As a result, the amount of adsorbed ammonia of the NOx selective reducing catalyst becomes saturated, therefore causing the problem of the ammonia generated each time the air-fuel ratio of the exhaust gas is made rich passing straight through the NOx selective reducing catalyst and being exhausted into the atmosphere.